A new communication method for enhancing the communication speed of radio communication with use of high-frequency electromagnetic waves called millimeter waves is currently under development. The wavelength of millimeter waves is 10 mm to 1 mm and the frequency thereof is 30 GHz to 300 GHz, and assignment of a channel in units of GHz is feasible in a 60-GHz band or the like, for example.
Generally, millimeter waves have characteristics that they propagate more straightly and are attenuated by reflection more significantly compared to microwaves. Therefore, a radio communication path in millimeter-wave communication are mainly direct waves or reflected waves reflected once or so. Further, millimeter waves also have characteristics that a free space propagation loss is large (reachable distance of the electric wave is short). Therefore, while radio communication using millimeter waves has an advantage that space division can be performed easier than the case of using microwaves, there is an aspect that a communication distance is short.
In order to compensate for such a weakness of millimeter waves and make use of high-speed radio communication using millimeter waves in a larger variety of scenes, one approach is to add a directionality to antennas of transmitting and receiving devices and aim a transmitting beam and a receiving beam in the direction where a device at the other end of communication is located to lengthen a communication distance. The directionality of a beam can be controlled by mounting a plurality of antennas on transmitting and receiving devices and assigning different weights to the respective antennas, for example. In the following Patent Literature 1, for example, a technique of performing radio communication with millimeter waves after exchanging a control signal through a communication medium such as sound waves, infrared rays, or light and learning an optimum antenna directionality is disclosed.